MHB Evaluating $m^4-18m^2-8m$ Given $p,\,q,\,r$

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The discussion focuses on evaluating the expression $m^4-18m^2-8m$, where $m=\sqrt{p}+\sqrt{q}+\sqrt{r}$ and $p,\,q,\,r$ are the roots of the polynomial $x^3-9x^2+11x-1=0$. The roots can be determined using Vieta's formulas, which provide relationships between the coefficients and the roots of the polynomial. The evaluation of the expression involves substituting the calculated value of $m$ into the polynomial expression.

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Let $p,\,q,\,r$ be the roots of $x^3-9x^2+11x-1=0$ and let $m=\sqrt{p}+\sqrt{q}+\sqrt{r}$.

Evaluate $m^4-18m^2-8m$.
 
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Re: Find m^4-18m^2-8m

anemone said:
Let $p,\,q,\,r$ be the roots of $x^3-9x^2+11x-1=0$ and let $m=\sqrt{p}+\sqrt{q}+\sqrt{r}$.

Evaluate $m^4-18m^2-8m$.

We have

$\displaystyle m^2 = p + q + r + 2(\sqrt{pq} + \sqrt{qr} + \sqrt{rp}) = 9 + 2(\sqrt{pq} + \sqrt{qr} + \sqrt{rp})$.

Thus

$\displaystyle (m^2-9)^2 = 4[pq + qr + rp + 2(\sqrt{p(pqr)} + \sqrt{q(pqr)} + \sqrt{r(pqr)})]$
$\displaystyle = 4[11 + 2(\sqrt{p} + \sqrt{q} + \sqrt{r})]$
$\displaystyle = 44 + 8m$.

Since $(m^2 - 9)^2 = m^4 - 18m^2 + 81$, we obtain $m^4 - 18m^2 + 81 = 44 + 8m$, or

$\displaystyle m^4 -18m^2 - 8m = -37$.
 
Last edited:
Awesome, Euge! Thanks for your solution and thanks for participating!:)
 

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